This invention relates to a fault detecting system for a control system forming a closed loop, and more particularly to a fault detecting system suitable for the speed control apparatus of an elevator.
In an elevator, any fault in its speed control system is very dangerous because it can cause an abnormal overspeed state to occur. Protective means for preventing an accident resulting in injury even if the overspeed state occurs, is therefore provided. In addition to this, devices are provided for detecting the fault of the speed control system before the occurrence of the overspeed state and preventing the overspeed state from occurring.
A typical construction for controlling the speed of an elevator is essentially that of a basic machine control system for controlling speed of a machine. This basic system compares a speed command signal and a feedback signal representative of the actual speed of the machine, and puts the deviation between these signals into a speed control unit. Even in a case where the controlled variable is not the speed but rather an electric quantity such as voltage and current, many of the controlling systems form a closed loop in which a command signal representative of the desired value of the controlled variable and a feedback signal representative of the actual value of the controlled variable are compared and in which the deviation between the signals is entered into a control unit.
One problem with control system forming a closed loop in this manner is faults which can be caused by an abnormality of a feedback circuit or an abnormality of the control apparatus proper.
Examples of prior systems for detecting such faults are described in the official gazettes of Japanese Patent Application Public-disclosure Nos. 48-58279 and 49-124756, which will now be briefly explained. In general, a feedback signal has a lag attributed to a control unit with respect to a command signal. Therefore, the feedback signal is put into a simulator circuit which includes a leading element of an inverse function to the transfer function of the control unit. The output of the simulator circuit is compared with the command signal. With this measure, when the control system is normal, the output signal and the command signal to be compared exhibit almost no difference, whereas when any fault is involved in the control system, a great difference is made between both the signals. In response to the fact that the difference has exceeded a predetermined value, the fault of the control system is detected.
It is very difficult, however, to fabricate a simulator circuit which accurately simulates the inverse function to the transfer function of the control unit. A great deal of time is required for adjustment, and even with this, a precise adjustment rarely be established.